UV curable powder coating compositions for improved through-cure

ABSTRACT

UV curable powder coating compositions containing 5 to 60 wt % spherical or near spherical particles having a median diameter of greater than 10, and most preferably greater than 15 microns, which refract and scatter applied UV light in all directions through the coating layer during curing to enhance the through-cure properties of the resulting coating.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patent application Ser. No. 09/717,413 filed Nov. 21, 2000, herein incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to ultraviolet light (UV) curable powder coating compositions and in particular to the addition of light-refractive microbeads to a colored UV-curable powder coating composition to improve the through-cure properties of the coating on curing with UV light.

BACKGROUND OF THE INVENTION

[0003] Colored powder coatings which can be cured by ultraviolet (UV) light have been under development for many years. Typically, these compositions contain an ethylenically unsaturated binder, a pigment or dye, and a specific photoinitiator. It is known that photoinitiators are added before irradiation in order to accelerate the photocuring of colored compositions. This renders it possible to cure such compositions, for example paints, in a very short time of irradiation sufficiently for their surface to be no longer tacky. While there are a number of technically satisfactory photoinitiators for transparent, i.e., clear, powder coating compositions, the radiation curing of colored compositions constitutes a problem particularly difficult to solve because of the presence of the light-absorbing pigments or dyes. In the case of paints used as exterior finishes for heavy machinery such as heavy duty trucks and farm vehicles there is also the requirement for extremely short curing times because of the high speed of modern assembly lines.

[0004] Various attempts have been made to use synergistic mixtures of photoinitiators which absorb light at different wavelengths not completely absorbed by the pigments to improve the curing of such colored compositions. However, none of these compositions have the ability to rapidly cure through their entire desired thickness. While adequate surface cure is usually attained, light cannot penetrate deeply enough in sufficient intensity to initiate adequate cure through the entire thickness of the coating. This is oftentimes referred to as poor or inadequate “through-cure”, which means the coating cannot be crosslinked or cured through the desired entire thickness. Typical consequences of inadequate through-cure are poor adhesion of the coating to the substrate and poor resistance to solvents. There is a great need to improve the through-cure properties of UV curable powder coatings.

[0005] The novel powder coating composition of this invention has the aforementioned desirable characteristics.

SUMMARY OF THE INVENTION

[0006] The invention relates therefore to a UV curable powder coating composition, wherein the improvement is the use in the composition of spheroidal particles, having a mean particle size greater than 10 microns and preferably greater than 15 microns, and having a maximum particle size of about 50 microns, which refract and scatter applied UV light in all directions through the coating layer during curing to enhance the through-cure properties of the resulting coating. In one particular embodiment, the improvement is the use of light-refractive glass or plastic microbeads in the composition for improved through-cure properties.

[0007] By “through-cure”, it is meant that enough ultraviolet light is able to penetrate the coating to initiate adequate crosslinking or curing through the entire thickness of the coating.

[0008] Articles comprising one or more layers of these coating materials are also included in this invention.

DETAILED DESCRIPTION OF THE INVENTION

[0009] The present invention is based upon the discovery that the through-cure properties of heat-fusible, UV curable powder coating compositions can be improved by formulating such powder coating compositions as compositions containing translucent, preferably optically-clear, light-refracting spheroidal particles, preferably glass or polymer microbeads, to cause UV light applied to the coating surface during curing to be refracted and scattered throughout the entire coating layer. As a result, sufficient UV light is able to penetrate through the entire coating thickness and enhance the curing of the coating down near the substrate. The light-refractive, clear or translucent beads in essence function as light diffusers within the coating layer to scatter light in all directions through the coating layer, to enhance the through-cure properties of the paint layer on curing.

[0010] The most critical component of the present compositions is the light-refracting spheroidal particles. The coatings of the present invention comprise translucent, preferably optically-clear, light-refracting spheroidal particles. The term “spheroidal” as used herein means generally spherical in shape. More specifically, the term means filler materials that contain less than 25% particle agglomerates or fractured particles containing sharp or rough edges. The spheroidal particles should be non-reactive or inert so as not to interfere with the other properties of the composition. Examples of suitable spheroidal particles are glass microspheres, naturally-occurring or synthetic spheroidal minerals such as cristobalite, and polymer microspheres. However, spherical glass and heat-resistant polymer microspheres, also referred to herein as “microbeads”, are generally preferred.

[0011] Through-cure depends on color because certain pigments absorb the wavelengths of light needed to initiate cure. Through-cure also depends on the type and amount of pigment blends and polymers present in the composition, because some combinations are especially effective at preventing the penetration of UV light. The light-refractive spheroidal particles must therefore be UV light transmissive and are preferably UV light transmissive at wavelengths not completely absorbed by the pigments or other materials present in the composition, and at wavelengths useful to activate the cure photoinitiators. For example, most UV light curable powder coatings are formulated from organic polymers which strongly absorb wavelengths below 280 nm. The extremely widely used white pigment titanium dioxide absorbs wavelengths below 400 nm. Useful cure photoinitiators absorb and react to light between about 280 nm and about 440 nm. Accordingly, desirable materials are those which substantially transmit light in the wavelength region between about 280 and 440 nm. Light transmission at wavelengths between 400 and 440 nm is especially desirable. Although the electromagnetic spectrum is a continuum, the standard convention is to divide ultraviolet light from the far-blue visible at the wavelength of 380 nm. The useful photo-curing wavelengths between 380 and 440 nm are thus usually classed in the blue-visible. For simplicity in this disclosure, the entire range between about 280 and about 440 nm is included in the ultraviolet range.

[0012] As already mentioned, the spheroid particles must have a mean particle size greater than 10 microns, preferably of greater than 15 microns. Intermediate ranges are included. As the mean particle diameter decreases, the surface per unit weight increases. The increase in surface area results in a tendency of the filler to dry the coating, reduce flow, and induce roughness in the coating. The effect of particle size on flow and roughness is more fully demonstrated in the parent application, U.S. application Ser. No. 09/717,413 filed Nov. 21, 2000 entitled “Low Gloss Powder Coatings,” the entire disclosure of which is incorporated herein by reference.

[0013] The upper limit of the diameter of the spheroidal particles is dependent on the intended thickness of the final coating in that the particles must have a diameter less than the coating thickness. Most powder coatings, especially “decorative” powder coatings, are designed to be applied at a dry film thickness of about 50 microns. Thus, in most applications, the spheroidal particles should have a maximum diameter of less than about 50 microns, preferably 40 microns.

[0014] The spheroidal particles may be present in the composition in an amount of from 5 wt % to 60 wt %, based on the total weight of the powder coating composition. Below 5 wt %, little effect is observed. Above 60 wt %, an unacceptable loss of coating flow results. It is understood that these are general guidelines and the exact weight % of spheroidal particles will depend on the specific gravity of the spheroidal particles, the degree of through cure desired, also the degree of gloss reduction desired (as explained below), and the other components of the powder coating composition.

[0015] The powder coating compositions of this invention also contain one or more resins commonly used in such coatings and well known in the art. Such resins include those based on epoxy, polyester, acrylic and/or urethane resins. Examples of such photopolymerizable resins include unsaturated polyesters, unsaturated acrylics, acrylates, unsaturated polyester-urethanes, unsaturated acrylic-urethanes, epoxies, acrylated epoxies epoxy-polyesters, polyester-acrylics, and epoxy-acrylics.

[0016] In addition to the resins and spheroidal particles, the powder coating compositions of this invention may contain other additives that are conventionally used in powder coating compositions. Examples of such additives include fillers, extenders, flow additives, photoinitiators, catalysts, hardeners, dyes and pigments. Compounds having anti-microbial activity may also be added as is taught in U.S. Pat. No. 6,093,407, the entire disclosure of which is incorporated herein by reference.

[0017] The powder coatings of this invention are prepared by conventional manufacturing techniques used in the powder coating industry. For example, the ingredients used in the powder coating, including the spheroidal particles, can be blended together and heated to a temperature to melt the mixture and then extruded. The extruded material is then cooled on chill rolls, broken up and then ground to a fine powder.

[0018] The spheroidal particles may also be combined with the coating powder after it is formed in a process known as “bonding.” In this process, the coating powder and the material to be “bonded” with it are blended and subjected to heating and impact fusion to join the differing particles.

[0019] The powder coating compositions of this invention may be applied by electrostatic spray, thermal or flame spraying, or fluidized bed coating methods, all of which are known to those skilled in the art. The coatings may be applied to metallic and/or non-metallic substrates. Following deposition of the powder coating to the desired thickness, the coated substrate is typically heated to melt the composition and cause it to flow. In certain applications, the part to be coated may be pre-heated before the application of the powder, and then either heated after the application of the powder or not. Gas or electrical furnaces are commonly used for various heating steps, but other methods (e.g., microwave) are also known. Curing (i.e., cross-linking) of the coating may then be a carried out by photochemical methods (i.e., ultraviolet radiation).

[0020] The powder coatings of this invention not only provide the formulator with an opportunity to control through-cure of the final coating, but also with the opportunity to control (i.e., reduce) gloss of the final coating while minimizing or eliminating the negative effects of the prior art attempts at controlling gloss; i.e., loss of coating flow and creation of “orange peel” surface effects.

[0021] In certain applications, it is necessary or desirable for the powder coating to have a surface that is smooth in appearance, but has a low gloss or shine. Such applications are those where low gloss is aesthetically desired, or where glare from the coating surface can interfere with the safe or proper use of the coated article, such as firearms, optical devices, military applications and motor vehicles, aircraft and other vehicles. It is important to note that the coatings of this invention have a rough or textured surface microscopically which is seen as low gloss, but otherwise appear smooth to the naked eye.

EXAMPLES

[0022] Table 1 identifies a number of commercially available spheroidal particles and characterizes their optical properties. TABLE 1 SPHEROIDAL PARTICLES MEDIAN OPTICAL GRADE MAX. DIA. (μM) DIA. (μM) PROPERTIES Glass Microspheres (Potters Industries, Inc, Valley Forge, PA) Spheriglass ™ 3000E 90% ≦ 65 μm 35 Clear¹ Spheriglass ™ 3000E 45 23 Clear screened at 45 μm Spheriglass ™ 90% ≦ 60 μm 35 Clear¹ 3000A Cristobalite (C.E.D. Process Minerals, Inc., Akron, OH) Goresil ™ C-325 95% ≦ 56 μm 12 Translucent¹

Examples 1-4 and Comparative Examples 1-5

[0023] The following examples illustrate the usefulness of the above spherical particles as through-cure control agents in UV-curable powder coating compositions. Specifically, the spheroidal fillers listed in Table 3 were tested in the composition listed below in Table 2: TABLE 2 UV-CURED UNSATURATED POLYESTER COMPOSITION Component Parts by weight Uvecoat ™ 2000 Unsaturated Polyester Resin (UCB) 100 Irgacure ™ 819 Photocuring Agent (Ciba) 1.85 Irgacure ™ 2959 Photocuring Agent (Ciba) 1.85 Troy EX570 Flow Aid (Troy) 1.2 Ti-Pure ™ R-706 TiO₂ Pigment (DuPont) See Table 3 Raven ™ 450 Carbon Black Pigment (Columbia) See Table 3 Spheroidal Particle See Table 3

[0024] Powder coating compositions were prepared by combining and bag-blending the components, followed by melt-extrusion. Extrudate was solidified between chilled rolls, then broken up and ground to powder. Powders were scalped at 80 mesh (180 microns) to remove coarse particles.

[0025] Coatings were prepared by applying the powdered compositions to 0.032 inch (0.081 cm) thick grounded steel panels using an electrostatic spray gun, then by baking the powder-coated panels for 5 minutes at 250-300° F., and subsequently passing the heated panels beneath two UV lamps, a Fusion “V” lamp and a Fusion “H” lamp for a total dose of approximately 2.5 J/cm² to yield a cured coating. All of the coatings had desirable appearance and smoothness The thickness of the powder coatings was approximately 50 microns.

[0026] After cooling, the coatings were evaluated for through-cure. These results appear in Table 3. TABLE 3 COATING COMPOSITIONS Parts by Weight Spheroidal Comparative Examples Examples Particle C1 C2 C3 C4 C5 1 2 3 4 Pigment¹  5 10 20 10 20 10 10 10 10 Screened² — — — — — 20 50 10 — Spheriglass ™  0 3000E Goresil C-325 ™ — — — — — — — — 50 Barium Sulfate — — — 68 27 — Properties Through-Cure³ 180 71 37 71 39 103  135  135  108  (μm) # (e) Transfer the film to a fresh 100 ml portion of MEK and soak an additional 5 minutes. Note that soaking in MEK does not affect the cured portion of the film from the upper surface, but dissolves and removes the exposed deeper areas of the film to which light was unable to penetrate, and which, in consequence, are not cured. (f) Remove the film from the MEK and allow to dry. (g) Measure film thickness and record as through-cure.

[0027] Discussion of Results

[0028] Comparative Examples 1-3. These three coatings show a predictable reduction in through-cure with increasing pigment loading.

[0029] Comparative Examples 4 and 5. These examples, when compared to Comparative Examples 2 and 3 show that standard powder coating fillers such as barium sulfate do not improve through-cure.

[0030] Examples 1-4 These examples show that the addition of ultraviolet light transparent particles to systems which have moderate through-cure of 71 μm increases through-cure (to 103, 135, 135, and 108 μm respectively). Comparison of Examples 2 and 4 both of which contained 50 parts of the light-refractive particles show that different particles give different enhancement. The borosilicate glass beads (Spheriglass™ 3000E) increased the through-cure from 71 to 135, and spheroidal crystalline silica (Goresil™ C-325) increased through-cure from 71 to 108. Comparative Example 2 and Examples 1 and 2 show that through-cure is increased as the loading of the particles is increased from 0 to 50 parts. Above 50 parts, no added benefit is seen.

[0031] The conclusion from these examples is that properly sized spherical fillers can be reliably used to improve through-cure in UV radiation cured powder coatings. 

What is claimed is:
 1. An ultraviolet radiation curable powder coating composition wherein the improvement is the use in the composition of light-refractive spheroidal particles, wherein the spheroidal particles comprise 5 to 50 wt % of the coating composition and wherein said spheroidal particles have a median particle diameter of greater than 10 microns and have a maximum particle diameter of about 50 microns.
 2. The coating composition of claim 1, wherein the spheroidal particles have a median diameter of greater than 15 microns.
 3. The coating composition of claim 1, wherein the spheroidal particles are selected from the group consisting of glass microspheres, spheroidal minerals, and polymer microspheres.
 4. An ultraviolet radiation curable powder coating composition containing a particulate blend of (a) at least one ethylenically unsaturated, photopolymerizable resin, (b) a pigment or a dye, and (c) a photoinitiator, wherein the improvement is the use in the composition of light-refractive glass or polymer microbeads, which refract and scatter applied UV light in all directions through the coating layer during curing.
 5. The coating composition of claim 4, wherein the glass or polymer microbeads have a median diameter of greater than 10 microns.
 6. The coating composition of claim 5, wherein the glass or polymer microbeads have a maximum diameter less than the thickness of the cured coating layer.
 7. The coating composition of claim 6, wherein the glass or polymer microbeads comprise 5 to 50 wt % of the coating composition.
 8. The coating composition of claim 7, wherein the at least one photopolymerizable resin is selected from the group consisting of: unsaturated polyesters, unsaturated acrylics, acrylates, unsaturated polyester-urethanes, unsaturated acrylic-urethanes, epoxies, acrylated epoxies epoxy-polyesters, polyester-acrylics, and epoxy-acrylics.
 9. A process of improving the through-cure of a ultraviolet radiation curable powder coating, comprising the step of adding to a powder coating composition between 5 and 60 wt %, based on the weight of the composition, of spheroidal particles having a median particle diameter of greater than 10 microns and a maximum diameter of about 50 microns.
 10. The process of claim 9, wherein the spheroidal particles have a median diameter of greater than 10 microns.
 11. The process of claim 9, wherein the spheroidal particles have a median diameter of greater than 15 microns.
 12. The process of claim 9, wherein the spheroidal particles are selected from the group consisting of glass microspheres, spheroidal minerals, and polymer microspheres. 